How to Use Must, Might, Could and Can’t
Have you ever said:
- He must be tired.
- She might be at home.
- That can’t be true!
These are called modals of deduction — we use them to make logical guesses based on evidence.
If you want to sound more natural and confident in English, mastering these modals is essential.
📘 What Are Modals of Deduction?
Modals of deduction are used when we guess something based on evidence.
We use:
- Must → strong certainty
- Might / Could → possibility
- Can’t → strong negative certainty
1️⃣ MUST (Strong Certainty – 90–100%)
Use must when you are almost sure something is true.
🔹 Structure
Must + base verb (present)
Must have + past participle (past)
✅ Examples (Present)
- She studied all night. She must be tired.
- The lights are on. They must be home.
✅ Examples (Past)
- He didn’t answer his phone. He must have forgotten.
- She got 95%. She must have worked hard.
💡 “Must” = logical conclusion based on evidence.
2️⃣ MIGHT / COULD (Possibility – 40–70%)
Use might or could when something is possible but not certain.
🔹 Structure
Might/Could + base verb
Might/Could have + past participle
✅ Examples (Present)
- She might be sleeping.
- He could be at the office.
✅ Examples (Past)
- They might have missed the bus.
- He could have misunderstood the instructions.
💡 “Might” and “Could” are very similar in meaning.
3️⃣ CAN’T (Strong Negative Certainty – 0%)
Use can’t when you are sure something is NOT true.
🔹 Structure
Can’t + base verb
Can’t have + past participle
✅ Examples (Present)
- That can’t be right.
- She can’t be serious.
✅ Examples (Past)
- He can’t have done that.
- They can’t have finished already.
💡 “Can’t” expresses strong disbelief.
🧠 Comparison Chart
| Modal | Meaning | Example |
|---|---|---|
| Must | Almost certain | She must be at home. |
| Might | Possible | She might be at home. |
| Could | Possible | She could be at home. |
| Can’t | Impossible | She can’t be at home. |
⚠️ Common Mistakes Learners Make
❌ She must to be tired.
✔ She must be tired.
❌ He might has left.
✔ He might have left.
❌ They can’t have went.
✔ They can’t have gone.
🎯 Improve Logical English with Sujyoti
At Sujyoti Language School, we teach grammar not as memorisation—but as real communication.
Our CEFR-based courses (A1–C2) help you master:
- Modals of deduction
- Advanced sentence structures
- Academic and professional English
- British pronunciation & fluency
- Weekly one-to-one exams
👉 Ready to upgrade your English?
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